Wireless transmitters can be used to transmit wireless signals via a wireless medium, and wireless receivers can be used to receive and recover transmitted wireless signals from the wireless medium. During transmission over the wireless medium, wireless signals may be distorted and/or otherwise degraded. This distortion and/or degradation may occur as a result of channel impairments such as noise, signal interference, intersymbol interference, co-channel interference, etc. To mitigate the effects of any actual and/or perceived channel impairments, some wireless transmitters code and/or otherwise process information contained in the transmitted wireless signals.
The structure of wireless transmitters and/or wireless transmitted signals can vary based on the wireless communication standard supported by the host wireless communications device. For example, IEEE 802.11a defines an orthogonal frequency division multiplexing (OFDM) wireless transmission protocol that comprises eight 20 MHz spaced channels in the lower band (e.g., 5.15 gigahertz to 5.35 gigahertz) and five 20 MHz spaced channels in the upper band (e.g., 5.725 gigahertz to 5.825 gigahertz). The analog and radio frequency (RF) sections of a conventional wireless transmitter include a baseband analog filter, an up-conversion mixer, a power amplifier driver (PAD), external filters, and a power amplifier (PA) capable of driving an antenna.
Measurement and control of RF power is a critical consideration when designing a wireless transmitter. Various factors, such as regulatory requirements on power transmitted, network robustness, and the need to co-exist with other wireless networks demand that there be tight control of transmitted power. Moreover, precise RF power control can result in improved spectral performance and can save cost and energy in the transmitter's power amplifier. Wireless standards such as Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE) require the wireless transmitter to operate at a specified transmit power (e.g., 23 dBm), with some allowance for output power variation as a function of device variation and load conditions (e.g., +/−2 dB). For device operation in the field, lower output power causes degradation of the link quality between the mobile station and the base station due to reduced signal-to-noise ratio (SNR) and hence range of operation. Conversely, if the power varies above the rated power of the PA, the signal quality is degraded due to non-linearities in the PA. Increased non-linearity causes signal degradation in error vector magnitude (EVM) which degrades link quality, or spectral emissions mask (SEM), which may result in violation of specifications (e.g., emission specifications).
The transmit power at the antenna of a wireless transmitter can vary due to several factors. For example, differences in gain among stages in a wireless transmitter can impact transmit power. While the nominal gain of each of the stages in a wireless transmitter may be known, the analog components (e.g., filters, mixers, power amplifier drivers, and power amplifiers) of the transmitter can have a significant variation in gain from part to part, and temperature effects can also impact these gain differences. RF transmitters are designed to support a span of RF frequencies (e.g., in WiMAX 2300 MHz to 2700 MHz). The gain of RF stages can vary as a function of the frequency at which the device is operated.
In addition, external filters on the board can filter out transmit noise in adjacent bands. These filters are usually designed to have a flat gain in the intended passband of operation (e.g., 2500 to 2700 MHz), while suppressing noise in the neighboring bands (e.g., 2400 to 2500 MHz). These filters have gain variation (e.g., 2 to 3 dB) due to ripple in their intended pass band. Another important factor impacting PA gain variation is the load presented by the antenna to the PA, which can vary as a function of reflections seen at the antenna. To maintain a constant output power delivered by an RF transmitter to the antenna, some conventional transmitters use a feedback loop (e.g., using a power detector circuit and/or an RF coupler to couple a fraction of the power to the input of a power detector circuit).
Conventional transmitters have several deficiencies. For example, in a conventional transmitter having a feedback loop, the power detector transfer function is dependent on the signal characteristics of the particular signal being transmitted (e.g., a QPSK signal vs. an OFDM signal), and hence the power detector needs to be characterized over a variety of signal characteristics. A short sample of the output (e.g., 5 in duration) may not have the same statistical distribution of the signal over a full transmit frame. Hence, in order to sense the average output power the PA accurately, the power detector output needs to be averaged over a substantial portion of the transmit frame. In wireless standards such as LTE, the peak to average power ratio (PAPR) of the transmit signal is different for different periods in time, within one subframe. In systems that use an OFDM-like transmit signal (e.g., WiMAX-OFDM, LTE-SCFDMA), it is also desirable to have the output power accurately regulated during a cyclic prefix of the transmit signal to avoid inter carrier interference. Similarly, phase estimation of a transmit signal can be useful for some special transmitter architectures such as the Cartesian Feedback Amplifier or an advanced transmitter that is used in an uplink beam forming system. Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present disclosure as set forth in the remainder of the present application with reference to the drawings.
What is needed therefore are systems, apparatuses, and methods for fast and accurate gain and phase error estimation for wireless transmitters.
Features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.